Heretofore, a glass used for a glass plate (glass substrate) for various displays, particularly glass plates on which surfaces a metal or oxide thin film is formed, has been required to have the following characteristics:    (1) Not substantially containing alkali metal ions; because in the case where the glass contains an alkali metal oxide, alkali metal ions diffuse in the thin film, resulting in deterioration of film characteristics.    (2) Having a high strain point so that deformation of a glass and shrinkage (thermal shrinkage) due to structure stabilization of the glass can be minimized when exposed to high temperature in a thin film formation step.    (3) Having sufficient chemical durability to various chemicals used in semiconductor formation; in particular, having durability to buffered hydrofluoric acid (BHF: mixed liquid of hydrofluoric acid and ammonium fluoride) for etching SiOx or SiNx, a chemical solution containing hydrochloric acid used for etching of ITO, various acids (nitric acid, sulfuric acid, etc.) used for etching of an metal electrode, and an alkaline of a resist removing liquid.    (4) Having no defects (bubbles, striae, inclusions, pits, flaws, etc.) in the inside and on the surface.
In addition to the above requirements, the recent situations are as follows.    (5) Reduction in weight of a display is required, and the glass itself is also required to be a glass having a small density.    (6) Reduction in weight of a display is required, and a decrease in thickness of the substrate glass is desired.    (7) In addition to conventional amorphous silicon (a-Si) type liquid crystal displays, polycrystal silicon (p-Si) type liquid crystal displays requiring a slightly high heat treatment temperature have come to be produced (a-Si: about 350° C.→p-Si: 350 to 550° C.) and thermal resistance is required.    (8) In order to improve productivity and increase thermal shock resistance by increasing the rate of rising and falling temperature in heat treatment for preparation of a liquid crystal display, a glass having a small average thermal expansion coefficient is required.
On the other hand, small and medium-sized displays for mobile as typified by a smartphone have progressed in high definition and thus, the above demands have become more and more strict.
In addition, color unevenness caused due to the stress generated in the glass plate at the time when a display is fitted in a panel becomes problematic. In order to suppress the color unevenness, it is necessary to decrease the photoelastic constant of glass. For this, it is effective to decrease the concentration of B2O3 or increase the concentration of BaO in the glass. Alkali-free glass capable of realizing a low photoelastic constant by adjusting the composition in this manner has been proposed (for example, see Patent Documents 1 to 3).
On the other hand, in a field of small and medium-sized liquid crystal displays (LCD), organic EL displays (OELD), and particularly, portable displays such as a mobile, a digital camera and a mobile phone, reduction in weight and thickness of a display has become an important issue. For the purpose of realizing a further reduction in thickness of a glass plate, a process containing bonding an array color filter and then performing an 3 0 etching treatment on the surface of a glass plate to reduce the plate thickness (reduction in thickness of a plate), has been widely used.